I decided to take apart my MBA by myself to clean it up. I know that it is wrong — it was my decision and I knew what to do. I did everything very carefully. After I put everything back my MBA works fine, even better than before, BUT the machine's serial number is missing and sound has been disabled (there is no startup sound, no sound in headphones, and the built-in speakers do not show at all in system info).
I didn't replace a motherboard, just took the MBA apart and put it back together again. So I can't see how I could have damaged something on the motherboard that would explain the serial number disappearing or sound not working.
Resetting a new serial number is not a huge deal — just need to use Blank Board Serializer, but I can't understand why it is missing.
Could the two be related? What could have happened that explains this.